"We're seeing some real problems out there," said Dr. Daniel Fabricant, who heads the FDA's Division of Dietary Supplement Programs.

While most vitamins and supplements are not harmful -- and at least one vitamin brand was credited with an 8 percent reduction in cancer among men over 50 -- the industry is beset by repeated recalls, manufacturing problems and adverse reactions caused by tainted products, health experts and regulatory officials say. And there is little the FDA can do to improve the situation, Fabricant said, unless Congress legislates more regulatory authority for the agency.

Roughly half the U.S. population -- 150 million people -- consumes multivitamins, mineral tablets, weight-loss aids, herbal remedies, protein powders and a host of other products that fall under the vast rubric of dietary supplements.

Supplements, a $28 billion industry made up of about 450 U.S. companies, are deemed "food" by law and are not subject to the tough regulatory scrutiny of prescription drugs.

Sixteen nationwide recalls and warnings have been issued in the past month and a half, including vitamins that contained the risky steroids dimethazine, dimethyltestosterone and methasterone. More than 3,000 products were recalled nationwide last year.

Written product recipes at numerous supplement companies are nonexistent, Fabricant said, and many recipes -- known as master manufacturing records -- are apparently cobbled together when owners learn that government inspectors are on their way.

Worse, drums in which products are mixed are not always appropriately cleaned, Fabricant added, and in some firms these vessels are damaged from age and/or overuse. Debris left from previous batches sometimes winds up in newly made products, he said.

Too often, dangerous drugs of all kinds -- from male sexual enhancement compounds to weight-loss medications -- are turning up in vitamins and other supplements nationwide.

A report in the Journal of the American Medical Association in April noted that potent drugs are sometimes purposely added to supplements to increase strength, usually weight loss remedies and sleep aids.

Sibutramine, for example, which is now banned, causes weight loss but also can lead to heart attack or stroke.

Consumers also are put at risk, Fabricant said, by raw products from foreign sources because of pervasive pesticide usage abroad. Most of what he calls "the alphabet vitamins" -- A, B, C, D and E -- have provenance in China. The same is true of botanicals, Fabricant said, some of which are found to be tainted with pesticides.

"What we're finding is that people (manufacturers) are not testing their products," Fabricant said.

Approximately 6,300 people nationwide complained about adverse reactions to dietary supplements between 2008 and 2012, according to FDA statistics. But the actual number may be more than eight times higher, some experts say, because most people don't believe health products can make them sick.

The FDA began inspecting how vitamins and other dietary supplements are manufactured only in 2008. And there is little the FDA can do to exercise more power over supplement safety without an act of Congress, Fabricant said.

U.S. Senate lawmakers reintroduced a measure this month to grant the agency more power. The bill was initially introduced in 2011.

Cara Welch, senior vice president of scientific and regulatory affairs for the Natural Products Association, a trade group in Washington, D.C., noted "the industry is trying to comply with the FDA's GMP requirements," she said of good manufacturing practice rules.

"This is an industry with a relatively new regulatory framework," she said.

Here is a list of recent FDA warnings, recalls and seizures:

July 19 -- Recall of Herbal Give Care LLC's weight loss and vitamin supplements. The products contain sibutramine, which is banned for heart attack risks, and two derivatives.